1. Field of the Invention
This invention relates to a wet flue gas desulfurization system wherein flue gas from thermal electric power plants and the like is brought into contact with an absorbent slurry whereby sulfur dioxide present therein is absorbed and removed. More particularly, it relates to a wet flue gas desulfurization system using an improved method for supplying an absorbent slurry to the absorption tower.
2. Description of the Related Art
In recent years, wet flue gas desulfurization systems of the commonly referred to in-situ oxidation type have become popular. In these systems, the necessity of an oxidation tower is eliminated by supplying air to the tank of an absorption tower so that an absorbent slurry (usually containing a calcium compound such as limestone) having sulfur dioxide absorbed therein may be oxidized by contact with air to form gypsum as a by-product. FIG. 4 is a schematic view illustrating an example of a wet lime-gypsum desulfurization system of this type.
As illustrated in FIG. 4, this system is equipped with an air sparger 3 of the commonly referred to rotating arm type which blows oxidizing air in the form of fine bubbles while agitating the slurry in a tank 2. Thus, the absorbent slurry having sulfur dioxide absorbed therein is brought into efficient contact with the air within tank 2 to be oxidized completely and thereby form gypsum.
More specifically, in this system, untreated flue gas A is introduced into a flue gas inlet section 1a of an absorption tower 1 and brought into contact with an absorbent slurry injected from header pipes 5 by means of a circulating pump 4 whereby sulfur dioxide present in untreated flue gas A is absorbed and removed. The resulting flue gas is discharged as treated flue gas B from a flue gas outlet section 1b. The absorbent slurry injected from header pipes 5 flows downward through a layer of packing material 6 while absorbing sulfur dioxide, and enters tank 2 where it is oxidized by contact with a large number of air bubbles blown in and agitated by air sparger 3, and then undergoes a neutralization reaction to form gypsum. The predominant reactions occurring in the course of these treatments are represented by the following reaction formulas (1) to (3).
(Flue gas inlet section of absorption tower) EQU SO.sub.2 +H.sub.2 O.fwdarw.H.sup.+ +HSO.sub.3.sup.- ( 1)
(Tank) EQU H.sup.+ +HSO.sub.3.sup.- +1/2O.sub.2 .fwdarw.2H.sup.+ +SO.sub.4.sup.2-( 2) EQU 2H.sup.+ +SO.sub.4.sup.2- +CaCO.sub.3 +H.sub.2 O.fwdarw.CaSO.sub.4 .multidot.2H.sub.2 O+CO.sub.2 ( 3)
Thus, the slurry within tank 2 comes to have suspended therein gypsum and a small amount of limestone used as absorbent. In this case, the slurry is withdrawn through a pipe line branching off from the delivery pipe of circulating pump 4, and fed to a solid-liquid separator 7 where it is filtered and recovered as gypsum C having a low water content (usually of about 10%). On the other hand, the filtrate from solid-liquid separator 7 is conveyed to a filtrate tank 8, stored temporarily therein, and suitably supplied to a slurry preparation tank 10 as feed water for the preparation of an absorbent slurry by means of a pump 9.
Slurry preparation tank 10 has a stirrer 11, by which limestone (absorbent) introduced thereinto from a limestone silo 12 by means of a conveyor 13 and water supplied by means of the aforesaid pump 9 are mixed and stirred to form an absorbent slurry. The resulting absorbent slurry is suitably withdrawn by means of a slurry pump 14 and fed to the tank 2 of absorption tower 1.
During the operation of this slurry preparation tank 10, the amount of water introduced thereinto is regulated, for example, by means of a controller and a flow control valve (not shown). Moreover, limestone is supplied thereto in an appropriate amount corresponding to the amount of water introduced, by controlling the operation of the rotary valve 12a of limestone silo 12. Thus, slurry preparation tank 10 is maintained in such a state that an absorbent slurry having a predetermined concentration (for example, of about 20% by weight) is always stored on a level within certain limits.
In addition, make-up water (i.e., industrial water or the like) is suitably supplied, for example, to the aforesaid filtrate tank 8. Thus, the water gradually lost, for example, by evaporation in absorption tower 1 is made up for.
Moreover, in order to maintain a high degree of desulfurization and a high purity of gypsum during operation, the sulfur dioxide concentration in untreated flue gas A, and the pH and limestone concentration and like of the slurry within tank 2 are detected with sensors, and the feed rate of limestone and the feed rate of the absorbent slurry are suitably regulated by means of controllers (not shown).
Furthermore, during operation, the temperature of the slurry within the tank 2 of absorption tower 1 is steadily maintained at about 50.degree. C. owing to the heat of the flue gas and the heat of reaction evolved in the aforesaid reactions.
In the above-described conventional wet flue gas desulfurization system, a pit for the preparation of an absorbent slurry (i.e., slurry preparation tank 10) and components attached thereto (e.g., stirrer 11, slurry pump 14, and the pipe line extending from slurry pump 14 to tank 2) are required. Consequently, there has been a limit to the simplification of the equipment which tends to be more and more eagerly desired in recent years.
Accordingly, as a means for simplifying such systems, the present inventors have already proposed a dry feed mechanism in which limestone used as absorbent is conveyed from the silo to the absorption tower by pneumatic conveyance (as described, for example, in Japanese Patent Publication No. 56130/'90). This mechanism makes it possible to eliminate the pit for the preparation of an absorbent slurry.
However, even if this mechanism is employed, a compressor and a limestone conveying line for pneumatic conveyance are still required. Accordingly, there is a need for further simplification of the equipment.